The invention relates to a manufacturing process for rotary die cutting machines wherein tolerances may be predictably achieved. The process is more specifically directed to a cylinder rotary die cutter to provide extended die life. More particularly, the invention relates to a finish grinding process for the cylinders of a rotary die cutting machine.
Rotary die cutting machines for cutting or perforating webs of material are run at high speeds and provide for a die cylinder to contact the web of material between an adjacent, usually counter-revolving, anvil cylinder.
The smaller the manufacturing tolerances for rotary die cutters, the more uniformly the cutters will cut the web of product being fed to them.
With high speed cutters heat build-up can create uneven expansion of the cyliners. It is improtant to maintain the operating tolerance of the cylinders relative to each other and avoid the heat expansion problems. In this regard, by reducing the cylinder total indicated runout (TIR), the die life and running speeds of the die and anvil cylinders can be increased. By reducing the TIR, the interference between the die cylinder and anvil cylinder will be reduced thereby minimizing die deterioration and heat buildup so that the cutting quality on the web of material may be consistently achieved over a longer period of time.
Conventional manufacturing procedures for rotary die cutters require the use of high precision bearings in order to achieve a high predictable precision of the completed die cutter. This requires using Class 3 bearing, which provide the best predictable cylinder gap tolerance achieveable down to about 0.0006 inches. This tolerance error is simply the result of the use of the Class 3 bearings alone. The present invention provides a manufacturing process that will allow the use of less precise Class 2 or 4 bearings but will achieve a much higher predictable cylinder gap tolerance of 0.00008 inches and better. This is seven and one-half times more precise then the prior art use of Class 3 bearings. The major advantage of the present invention is the improved cylinder tolerance while using less precise bearings and greatly reducing cost. Shorter delivery times are also achieved by using the less precise bearings.
It is an allied goal of the invention to greatly increase the die life by providing a method for making die and anvil cylinders with very close TIR tolerances that reduce the gap variations between the cylinders. This goal thereby reduces the extent of die-to-anvil impingement or contact to only that which is requireed in order to maintain consistent cutting of the web of material passing between the cylinders. This reduction of interference will further eliminate much of the risk in developing destructive harmonic resonance during high speed operation.
It is a related object to provide a finishing process for rotary die cutter cylinders that makes possible considerably less frequent resurfacings of the anvil cylinder.
The invention may be summarized as a manufacturing process for making the die and anvil cylinders of a rotary die cutter which produces a predictable die-to-anvil gap tolerance in the completed rotary die cutting machine. The tolerance is several times more accurate than can be predictably achieved by prior art methods used for manufacturing rotary die cutters. The process involves the finish (cylindrical) grinding of the die and anvil cylinders by rotating the cylinders in the same preloaded bearings that are to be subsequently operated in the die cutting machine. Since the die and anvil cylinders, with preloaded bearing assemblies, are finish ground, they are kept in the assembled state and mounted in a die cutter side frame. Prior art processes for making die and anvil cylinders provide for the finish (cylindrical) grinding of the cylinders as they are rotated on centers or their bearing journals whereby there is no elimination of the bearing tolerances because the bearings are mounted separately after the finish grinding of the cylinders. Thus compounded errors of each machined tolerance results in conventional processes and the predictability of the tolerance of the completed machine is uncertain. The present invention overcomes this major obstacle found in the art.